The Swift GRB Host Galaxy Legacy Survey. Ii. Rest-Frame Near-Ir Luminosity Distribution and Evidence for a Near-Solar Metallicity Threshold

Abstract

We present rest-frame NIR luminosities and stellar masses for a large and uniformly-selected population of GRB host galaxies using deep Spitzer Space Telescope imaging of 119 targets from the Swift GRB Host Galaxy Legacy Survey spanning 0.03 < z < 6.3, and determine the effects of galaxy evolution and chemical enrichment on the mass distribution of the GRB host population across cosmic history. We find a rapid increase in the characteristic NIR host luminosity between z ∼ 0.5 and z ∼ 1.5, but little variation between z ∼ 1.5 and z ∼ 5. Dust-obscured GRBs dominate the massive host population but are only rarely seen associated with low-mass hosts, indicating that massive star-forming galaxies are universally and (to some extent) homogeneously dusty at high-redshift while low-mass star-forming galaxies retain little dust in their ISM. Comparing our luminosity distributions to field surveys and measurements of the high-z mass-metallicity relation, our results have good consistency with a model in which the GRB rate per unit star-formation is constant in galaxies with gas-phase metallicity below approximately the Solar value but heavily suppressed in more metal-rich environments. This model also naturally explains the previously-reported "excess" in the GRB rate beyond z 2; metals stifle GRB production in most galaxies at z < 1.5 but have only minor impact at higher redshifts. The metallicity threshold we infer is much higher than predicted by single-star models and favors a binary progenitor. Our observations also constrain the fraction of cosmic star-formation in low-mass galaxies undetectable to Spitzer to be small at z < 4.

Are long gamma-ray bursts biased tracers of star formation? Clues from the host galaxies of theSwift/BAT6 complete sample of bright LGRBs

“…Early studies (see Introduction) on the subject based on incomplete LGRB samples reported a strong preference towards low metallicity values. Lately, however, various studies have indirectly pointed out that this view is only partially correct Krühler et al 2015;Perley et al 2016b). Our results showed that at 0.3 < z < 1 LGRBs preferentially select galaxies of sub-solar metallicities (12 + log O H ∼ 8.4-8.5) and therefore of low stellar masses.…”

“…Our comparisons to VVDS and UltraVista surveys, taking the completeness in brightness, SFR, and M * of the samples into account, indicate that the metallicities of LGRB hosts and star-forming galaxies have similar distributions up to 12 + log O H ∼ 8.4-8.5, after which the already discussed paucity of high-metallicity hosts is observed. This cutoff value, obtained by direct comparison, is similar to the one found in an indirect way in completesample studies by Vergani et al (2015) and Perley et al (2016b). Finally, we note that there are two hosts with very low metallicities in the BAT6 sample with poorly constrained values, which limits their weight in the analysis.…”

“…LGRB hosts at z < 1 are also found to be fainter and of lower stellar mass than a field star-forming galaxy population (see also Vergani et al 2015;Perley et al 2013Perley et al , 2016b. Because the stellar mass and metallicity of star-forming galaxies are correlated (stellar massmetallicity relation, Tremonti et al 2004), the low-metallicity preference could provide the explanation for the differences in observed stellar masses between populations.…”

“…At high redshifts very few galaxies are used in analyses, which is presumably the reason why the conclusions drawn from different unbiased samples still differ: while some studies (Greiner et al 2015;Perley et al 2016b) claimed that LGRB hosts can be direct tracers of star formation at about z > 3, others find the hosts to be of low luminosity with a metallicitydependent efficiency (Schulze et al 2015).…”

“…Several studies have investigated the metallicity of hosts and its effect on GRB efficiency, especially since theoretical models for single LGRB progenitor stars have predicted a low metallicity threshold above which LGRBs could not occur (Yoon et al 2006;Woosley & Heger 2006). The recent evidence, either direct (Krühler et al 2015) or indirect Perley et al 2016b;Schulze et al 2015), 1 GRBs are traditionally classified as long and short according to their observed duration (i.e. longer or shorter than ∼2 s).…”